The conjugation of drugs to monoclonal antibodies or other polypeptides often results in the formation of a heterogeneous mixture of antibody drug conjugates (ADCs) bearing varying number of molecules per polypeptide. The capacity to attach molecules reliably and efficiently to one or more defined site(s) of the antibody is highly desirable in order to prepare chemically homogeneous compounds. Accordingly, there is a need in the art for new engineered binding polypeptides which allow for stoichiometrically constant conjugation of moieties.